Two-way communication system



Feb. 7, 1939. L. w. THoMsEN TWO-WAY COMMUNICATION SYSTEM Filed Oct. 1, 1956 2 Sheets- Sheet l ehm 1li lll Illll lll IIIJII l Ill IlllllllllllllllIJ INVENTOR. Zo W'TZSZ/ ATT EY Feb. 7, 1939. L w. THoMsEN 2,146,362

TWO-WAY COMMUNICATION SYSTEM Filed 0G12. l, 1956 2 SheQtS-Sheet 2 JWNHHH 111211-11--- w .lllllllli mwm,

l gli# :lullin mmm INVENTOR A RN Puentes Fehn, 1939 i UNITED sfTATEs PATENT oFFlc-E 2,146,362 Two-WAY coMMurucA'rIoN SYSTEM Lena w. Thomsen, washington, D. c.

Application October 1, 1936, Serial No. 103,585 13 calma (ci. 179-1) ThisV invention relates to electrical apparatus generally.. More particularly this invention relates to annunciator, public address and auditorium telephonie and loud-speaking systems in which an operator may call or speak to any one or group of remote points, through loud-speakers or similar reproducers, or in which the operator may be called from any one of a plurality of remote points at which said loud-speakers orv similar reproducers are positioned.

An object of this invention is to provide a simple telephonic communication system adapted to be used for delivering messages between a central office and one or more remote points at which reproducing devices are located.

Another object of this invention is to provide a simple telephonie communication system adapted to be used between an ofce and a plurality of points with which it is desired to communicate either selectively or as a group.

Still another object of this invention is to provide a system employing a microphone, an` am plifier and a loud-speaker at a central station and a plurality of loud-speakers positioned at remote points and adapted to be energized either singly or `as a unit from said amplifier, whereby the central station may address either one or more of said remote points at will and at the same time be able to receive messages over the central station loud-speaker from the remote loud-speakers.

A further object of this invention is to provide a system employing a central station in which is provided a microphone, an amplifier, a loud- Y speaker and a circuit selecting arrangement, and a plurality of remote stations each including a loud-speaker adapted to function both as a sound reproducer and a sound pick-up, said system be# ing arranged to operate as a selective message distribution system whereby announcements and the like may be made from the central station and reproduced at the remote stations and messages transmitted from the remote stations to the central station upon the completion of certain circuits in the central station.

Another object of this invention is to provide a switching mechanism to switch the input and output of an amplifier within a very limited space and still avoid audiol frequency and radio frequency feedback.

A further object of this invention is to provide a circuit making it possible to call the main station from any loud speaker substation when the amplifier system isA not otherwise in use or is inactive.

Still a further object of this invention is to provide a talk-listen switch at the main stationv to be used to divide the loud speaker system into two circuits, one for carrying on two way communication, and the other for calling-in purposes from speaker substation.

Another object of this invention is to provide two selector dials to make it possible to select any one oi' a possible hundred loud speaker substations.

Another object of this invention is to provide a system in which the contacts to the input and output of amplifier in the talk-listen switch are held at ground potential until the input and output of the amplifier' are connected to the desired circuit, after which the input and output of the amplifier, the master station speaker, andthe microphonel are disconnected from ground poten-` tial.

Another object of this invention is to provide a talk-listen switch having three positions, an up position; a neutral position; and a down position. When in the up position the system is in condition so that each substation speaker may call the main operator through an auxiliary calling circuit.- In the neutra position the amplifier system is disconnected from the auxiliary calling circuit and is open to only one substation depending on the position of the selector switch and the output of the amplifier is connected to the loud speaker of the main station. In the down position the amplifier is switched from a receiving condition to a transmitting condition to transmit to the same as it received from in the up" position.

Other and further objects of this invention will be apparent from the following specification and the claims.

In accordance with this invention I provide a message distribution system of a simple and novel character which is readily adapted to industrial use, for example, in factories, garages, laundries, and similar establishments, as well as in educational institutions, particularly where one or more individuals are to be addressed over a loudspeaking reproducing system.

The arrangement of my invention is flexible and readily adjustable and may be used with ease, because of the simplicity of its operation, by persons other than those familiar with the technical details thereof.

Fundamentally my invention consists of a central or control station at which is located a microphone, an ampliiier, a loud-speaker and a set of selector switches, a number of additional loudspeakers, which are remotely positioned around 50 cuits, making a total o! 100 loud speakers thatA invention as Y herein described.

various places of a factory, for" example, with which it is desired to communicate from the central point, are connected to the 'selector switch arrangement. Through this switch arrangement the loud-speakers at the remote points may be energized either singly or all together as a unit. Persons at the remote points may address the central station over these remote loud-speakers and a two-way, talklisten conversation may be carried on between a remote point and the central station.

Other features of my invention will be apparent from the following speciilcation and the drawings in which, briefly, Figures 1 and 2 illustrate the wiring diagram schematically; Fig. 3 is a detail view of the talk-listen switch; Fig. 4 is a detail view of the group-'switch and Fig. 5 is a detail View of the plug and receptacle loud speaker connector.

Referring to Figure 1 of the drawings in detail, reference numerals I to III inclusive designate plug receptacles, each of which is provided with twelve plug receiving socket units, as illustrated in more detail in Figure 5. Connections are provided between certain ones of the plug receiving sockets of the receptacles I to I0 inclusive, and the division selector switch contacts. This division selector switch is provided with switch umts II to inclusive, each of which has a rotatable contact arm journaled in the center thereof and adapted to engage the contacts a, b, c, d, e, f, g, y, i, i, provided to each unit. These units II to 23 inclusive are preferably tandem mounted in .such a manner that the rotatable contact arms may be rotated simultaneously by the dial 23. One of the plug receptacles contacts of each of the units I to I0 inclusive is grounded through a connection 22, 22a, 22h, 22c, 22d, and so on, as illustrated. An adjacent plug receptacle conl 40 tact of each of these units I to III is connected through a suitable resistance unit to the wire 2i so that this particular contact or plug receptacle of each of the units I to III, inclusive, is connected to a common circuit designated by the wire 2I which is connected to the primary of the transformer 24, shown as Figure 2 of the drawings. 'Ihe remaining contacts of each of the units I to I3 inclusive, that is, ten contacts of each of these units, are connected to loud speaker cirmay be employed with the arrangement of my These loud speakers are each connected in the same manner as illustrated by the connections to loud speaker 55 units 25a and 26a. Each of the loud speaker units is provided with a single pole double throw substation switch such as illustrated by reference numerals 21a and 23a in the cases of loud speakers a and 25a. These substation switches each in- 00 clude a movable contact and a pair of stationary contacts designated by A and B and are arranged in such a manner that the movable contact is always in engagement with the stationary contact A unless the operator causes this movable gg contact to engage the contact B. 'I'he movable two terminals. However, it is obvious that loud speakers with independent neld terminals provided to connect the loud speaker field to the enerf gizing circuit may be employed ii' desired. In the so-called moving coil type of 'loud speakers employing an additional winding for energizing the field, the moving coil winding must be connected to the circuit as shown in the cases of the loud speakers 25 and 23. It is, of course, obvious that the piezo-electric crystal type loud speaker and the permanent magnet ileld type of moving coil loud speaker as well as the telephone type of speaker may be employed in accordance with this invention.

The contacts A and B of the switches associated with each of the loud speakers, that is, the substation switches, are connected to the plug receptacle units I to I0 inclusive. For example, the contacts A and B of the switch 21 are connected to the unit I and the contacts A and B of the switch 23 are connected to the unit 2. In each case the contact B is connected with that unit of the group I to III to which the contact A is also connected. However, all of the contacts B are connected to the common line 2l for the purpose of providing each of the loud speaker substations with means whereby the operator at any one of the loud speaker substations may call the main station simply by throwing the substation switch, corresponding to a switch such as switch 21, into the B position. and call the main station simply by speaking into the loud speaker substation.

Only some of the interconnecting wires provided between the plug receptacle units I to IU inclusive and the division selector units Il to 20 inclusive, are shown in the drawings. However, a suillcient number of these interconnecting wires is illustrated so that the complete wiring diagram may be derived from that portion which is illustrated. As shown, a wire is provided between each of the units I to I0 inclusive and each of the units II to 20 inclusive, each of the movable contacts of the units II to 20 inclusive is connected to a separate contact of the station selector switch 25 shown in Figure 2. This station selector switch 25 is provided with a rotatable contacting arm and ten stationary contacts adapted to be engaged by the rotating contact. 'I'he rotating arm contact of the unit II is connected by wire 25 to contact I of switch 25. Likewise the contacting arms of the units I2, I3, I4, I5, I6, I1, I8, I9, and 20 are connected through wires 21, 23, 29, 3l, 3I, 32, 33, 34, and 35 respectively, to the contacts 2, 3, 4,- 5, 6. 1, 8, 9, and I0, respectively, of the station selector switch 25. The contacts I, 2, 3, 4, 5, 5, 1, 8, 9, and III of the station selector switch 25 are connected to the contacts I, 2, 3, 4, 5, 6, 1, 8, 9, and III, respectively, of the grouping switch 35. This grouping switch 33 is 'a short circuit switch which is employed for the' purpose of grouping the loud speaker stations in groups of five so that it is possible for the operator at the main station to talk to ve loud speaker stations at one time.

The rotatable contact of the station selector switch 25 is connected through the wire 31 to the switch arm R ofthe talk-listen switch 38. The microphone 39 of the main station is connectedr through a shielded conductor 40 to the contact P of the switch 33. The master loud speaker 4I of the main station is connected through the shielded conductor'42 to the contact E of the switch 3l.l

The movable contact E loi' the switch 33 is con- 75 nected through the shielded conductor 43 to the output transformer of the amplifier 44.

The amplifier 44 is of more or less conventional design employing four amplifier tubes coupled together by suitable circuits and energized from a suitable power supply 44a which comprises a transformer, a rectifier tube, and filter circuits, and is connected to the alternating current lighting circuit.

The talk-listen switch 38 is of a special construction and is provided with a control lever 38a pivoted at 38h in such a way that the roller 38C engages and manipulates two groups of contact arms E, R, L, and O in the top section and S and D in the bottom section of the switch made of resilient material, such as, brass, bronze, and the like. The roller 38C is preferably of insulating material such as Bakelite, hard rubber, ber, or the like, and presses one group of resilient contacts E, R, L, and O up when the lever 38a is pressed down or presses the group including S and D when the lever is pressed up. The lever 38a may be placed into three positions, an upper position known as the call-in position, a horizontal position known as the receiving or listen position, and a lower position which is called the talking position. When this lever 38a is in the horizontal or receiving position the resilient contacts E, R, L, and O are in engagement with the contacts elements F, V, K. U, respectively, and

the contacts S and D are in engagement with I" and N, respectively, as shown in Figure 2. When the lever 38a is pressed downward into the talking position, the resilient contacts E, R, L, and O are raised and contact is made with the upper row of contacts, namely. the contacts M, Q. W. and P, respectively. The circuit between the resilient contacts Sand D and the contacts I and N is not broken whenthe lever is pressed downward "into 'the talking position. When the lever 38a is pressed upward into call-in position, the resilient contacts S and D are brought downward and the contact D is brought into engagement with the contact C. At the same time the pressure is released on the contacts E, R, L. and O and these contacts are permitted to spring back into engagement with the contacts F,V, K, and U, respectively, while the lever 38a ls pressed upward into call-in position.

` When the communication system of this invention is not in active use, the operator of the main station having control of the talk-listen switch 38 leaves the lever 38 of the switch in the upper or call-in position. When the switch is in this position, it is possible for any person desiring to do so to call the main station from any of the loud speaker substations positioned at dierent parts of the building. In order to call the main station from a loud speaker substation, the substation switch corresponding to a switch such as 21 or 28 shown in Figure 1, must be pulled down into position to contact the contact B. This connects the particular loud speaker substation to the common line 2| and then the person at the loud speaker substation can give the operator at the main station a number of the substations so that the operator at the main station can adjust the divisionselector switch 23 and the station selector 25 to the number of the particular substation, after which a two-way communication can be held between this particular substation and the main station. A'fter the personr at the substation receives the attention of the operator at the main station and advises the operator at the main station of the number of the substation,

it is no longer necessary for the operator at the substation to maintain the substation switch on the contact B inasmuch as the main operator having dialed the substations number, establishes the desired connection through the division selector and station selector switches and associated circuits thereof.

When the talk-listen switch 38 is in the call-in position, it connects the input of the amplifier 44 to every loud speaker substation, that is, to the common circuit 2| and al1 that is necessary then is to connect the loud speaker substation switch to the B position in order to communicate with the main station. When the switch 38 is in the call-in position, the output of the amplier 44 is connected to a loud speaker 4I which is part of the main station equipment, and through which the operator of the main station hears the conversation of the person at the substation. A switch corresponding to switch 21 or 28 of a loud speaker substation is moved from the A contact to the B contact, then that particular loud speaker substation is connected to a common line 2l which is connected through transformer 24 to the contacts C and I of the talklistenswitch 38. In the call-in position of switch 38, the switch blade D contacts the terminal C and through this connection the input of the amplier 44 is connected by means of the wire 45 to the common line 2l. While this switch 38 is in the call-in position the output of the arnplier 44 is connected through the contacts E and F to the loud speaker 4l.

Provision is made in this talk-listen switch 38 to eliminate high frequency feedback between the various circuits connected thereto.

Shields G, H, I, and J of metallic material, such as, copper, aluminum. or the like, inclose the output section of the switch 38, that is, the section connected to the output of the amplifier and to the loud speaker 4l. While the switch 38 is in call-in position, contact K is grounded through the contact L, and contact M being connected to contact K is also at ground potendal.- Additional shields are provided in the switch 38 to shield the section thereof con.- nected to the microphone 39 and the section connected to the input oi the amplifier on the other portions of the switch; these shields are designated by X and Y.

When the switch 38 is in the talking position, the contacts thereof are arranged so that` the master station can carry on communication with any desired substation, depending upon the setting of the station selector and the division selector. For example, in the drawings the station selector is set on the position 2 and the division selector is set on the position B. Accordingly, the

station connected to the talk-listen switch 38y would be connected to the substation contact block 2b in Figure 1 inasmuch as the position 2 of the station selector is connected through the wire 21 to the rotatable arm of the switch l2. Inasmuch as the division selector 23 is set on position B. the rotatable arm of the switch I2 would be connected to the terminal B thereof and would accordingly select the loud speaker substation connected fto this line. In this fashion any one of the one' hundred substations connected to the terminal blocks I to l0 inclusive may be selected merely by manipulating the station selector and the division selector.

When the switch 38 isin the talking position, the circuits in this switch are' arranged as follows. The contact D is in engagement with the contact N which is connected to O and O is connected to the microphone contact P. All of these contacts are complet-ely shielded from the output of the amplifier. At the same time the contact U is at ground potential through the condenser C9 which is connected between the contacts U and V. This latter contact V is connected to W which is connected to ground through L to insure against feedback within the switch.

The output of the amplifier is connected to the contact E which is connected to the contact M while the switch 38 is in the talking position. Contact M is connected to contacts Q and K. Q, however, is connected to R and to the station selector switch 25 through the line 31 and the station selector is connected to the substation B2 as outlined above. The loud speaker substations are always connected to the terminal blocks I to I inclusive through the substation switches which are always on the A contact as heretofore described. Therefore these substations may be called through the circuits of these terminal blocks l to I0 inclusive when desired. These substation switches are placed in contact with the B contacts thereof when the substation desires to call the main station through the common line 2| as hereinbefore described.

When the switch 38 is in the listening position, the circuits thereof will be the same as shown in Figure 2 of the drawings. rI'he input of the amplifier is connected through the contact D to the contact N and to the contacts O and U. The contact U is connected to the contact V through the condenser C9 and through the circuit of the switch contact R and the line 31 to the station selector switch 25 which is connected to the substation B2 as described.

The output of the amplifier is connected to the master speaker through the contacts E and F when the switch 3B is in the listening position. At the same time the contacts M and Q are grounded through the contacts K and L, thus insuring against feedback in the switch.

'I'he group switch 36, shown in Figures 2 and 4 is a five-pole double throw switch and is used Yto connect the circuits of the station selector switch 25 so that groups of five substations may be communicated with at the same time particularly when an announcement is to be made from the master station to a group of five stations simultaneously. This may be accomplished by throwing the lever of the switch 36 either in one direction or the other. If this switch is thrown in one direction, then the contacts 36a and 36h connect terminals I, 2, 3, l, and 5 together and if thrown in the opposite direction the contacts 36a and 36h' connect terminals 6, 1. 8, 9, and I0 together.

While I have described this invention in4 detail with respect to an embodiment thereof, it is, of course, understood that modifications therein may be made within the scope and spirit of the invention and therefore I do not desire to limit the invention to the exact details described except insofar as ,they may be defined by the claims.

Whatv I claim and desire to secure by Letters Patent is:

1. A communication system adapted for installation in buildings and the like, comprising a main station including an amplifier, a microphone, a loud speaker, and a switching device connected to said amplifier, said microphone, and said loud speaker, a plurality of substation loud speakers located at different places about the building, a switching system including a plurality of switch units, connections between selected ones of said switch units and selected substation loud speakers, a substation selector switch, connections between each of said switch units and said substation selector switch, means for connecting said first mentioned switching device to said substation selector switch whereby communication may be established between said main station and any one of said substation loud speakers, a callin circuit connected between all of said substation loud speakers and said main station and a manually operable switch at each of said substations for connecting selected substation loud speakers to said call-in line when it is desired to gall the main station from the selected substaion.

2. A communication system adapted for installation in buildings and the like, comprising a main station including an amplifier, a sound pickup device, a receiver and means for connecting either said sound pickup device to the input circuit of said amplifier or said receiver to the output circuit of said amplifier, a plurality of remotely positioned loud-speaking devices adapted to function either as sound pick-up devices or as sound reproducers, means for connecting any one of said loud-speaking devices to the output circuit of said amplifier when said sound pick-up device is connected to the input circuit of said amplifier and for connecting any one of said loudspeaking devices to the input circuit of said amplifier to function as a sound pick-up device when said receiver is connected to the output circuit of said amplifier, a call-in circuit connected between all of said remotely positioned loudspeaking devices and said main station and a manually operable switch at each of said remotely positioned loud-speaking devices for connecting any selected one of said loud-speaking devices to said call-in circuit when it is desired to call-in the main station therefrom.

3. A communication system adapted for installation in buildings and the like comprising a plurality of substation loud speakers positioned at various places around the building with which it is desired to communicate from a central office, a main station located at the central office, said main station including a sound reproducing device, a sound pick-up device, an amplifier, and a connecting device connected to the input and output circuits of said amplifier and to said sound reproducing device and said sound pick-up device, means connected to said connecting device for selecting the substation with which it is desired to communicate and means in said connecting device for isolating and grounding circuits of said amplifier while the circuits to the input and output of said amplifier are being changed.

4. A communication system adapted for installation in buildings and the like, comprising a main station including an amplifier, a microphone, a loud speaker, and a switching device connected to said amplifier, said microphone, and said loud speaker, a plurality of substation loud speakers located at different places about the building, a calling circuit havingmeans for connecting each of said substation loud speakers thereto for calling said main station, a switching system including a plurality of tandem mounted switch units, connections between selected ones of said units and selected substation loud speakers lwhereby all of said substation loud speakers are connected to at least one of said tandem mounted switch units, a substation selector switch, connections between each of said tandem mounted switch units and said substation selector switch,

lil

and means for connecting said rst mentioned switching device to said substation selector switch whereby communication may be established between said main station and anyone of said substation loud speakers.

5. A communication system adapted for installation in buildings and the like, comprising a main station including an ampliiier, a sound pickup device, a receiver and means for connecting either said sound pick-updevice to the input circuit of said amplifier or said receiver to the output circuit of said amplier, a plurality of remotely positioned loud-speaking devices, a calling circuit having means for connecting each of said remotely positioned loud-speaking devices thereto for calling said main station, means for connecting any one of said loud speaking devices to the output circuit of said amplifier when said sound pick-up device is connected to the input circuit of said amplier and for connecting any one of said loud-speaking devices to the input circuit of said amplifier when said receiver is connected to the output circuit of said amplifier, said last means including means for preventing undesired interference between the input and output circuits of said amplifier caused by changing the'circuit connections thereto.

6. A communication system adapted for installation in ,buildings and the like, comprising a main station including an amplifier, a sound pickup device, a receiver and a talk-listen switch for connecting said sound pick-up device to the input circuit of said amplifier in the talking position and for connecting said receiver to the output circuit of said amplier in the receiving position, said switch having a third or call-in position, a plurality oi remotely positioned loudspeaking devices, means for connecting any one of said loud speaking devices to the output circuit of said amplifier when said switch is in the talking position, additional means for connecting each of said remotely positioned loud-speaking devices to said amplifier when said switch is in the call-in position, and means for connecting any one of said remotely positioned loudspeaking devices to the input circuit of said amplifier when said switch is in the receiving position, and means in said switch for preventing undesired interference between the input and output circuits of said amplier caused by changing the circuit connections thereto.

'7. A communication system adapted for installation in buildings and the like, comprising a main station including an amplifier, a sound pickup device, a telephonie receiver and a talk-listen switch for connecting said sound pick-up device to the input circuit of said amplifier in the talking position and for connecting said receiver to the output circuit of said amplifier in the receiving position, a plurality of remotely positioned loud-speaking devices, means for connecting any one of said loud speaking devices to the output circuit of said amplifier when switch is in the talking position, and means for connecting any one of said remotely positioned loud-speaking devices to the input circuit of said amplifier when said switch is in the receiving position, and means in said switch for preventing undesired interference between the input and output circuits of said amplifier caused by vchanging the circuit connections thereto.

8. A communication system adapted for installation in buildings and the like comprising a plurality of substation loud speakers positioned at various places around the building with which it is desired to communicate from a central office, a main station located at the central ofiice, said main station including a sound reproducing device, a sound pick-up device, an amplifier, and a talk-listen switch device connected to the input and output circuits of said amplifier and to said sound reproducing device and said sound pick-up device, means connected to said connecting device for selecting the substation with which it is desired to communicate, said talk-listen switch device including means for isolating and grounding circuits of said amplifier while the circuits to the input and output of said amplifier are being changed.

9. A communication system adapted for installation in buildings and the like comprising a plurality of substation loud speakers positioned at various places around the building with which it is desired to communicate from a central ofce, a main station located at the central oice, said main station including a sound reproducing device, a sound pick-up device, an amplier, and a connecting device connected to the input and output circuits of said amplifier and to said sound reproducing device and said sound pick-up device, and means connected to said connecting device for selecting the substation with which it is desired to communicate, a call-in line connected to each of said substation loud speakers and to said connecting device, said connecting device including means for isolating and grounding circuits of said amplifier while the circuits to the input and output of said amplifier are being changed, said connecting device including additional means for connecting said call-in line to said ampliiier for permitting said main station to be called from any one of said substations.

10. A communication system adapted for installation in buildings and the like, comprising a main station including an amplifier, a microphone, a loud speaker, and a talk-listen switch device connected to said amplier, said microphone, ad said loud speaker, a plurality of substation loud speakers located at different places about the building, a call-in line connected to all of said loud-speaker substations, means included in said talk-listen switch device for connecting said call-in line to said amplifier to permit any of said substations to call said main station, a switching system including a plurality of tandem mounted switch units, connections between selected ones of said units and selected substation loud speakers whereby all of said substation loud speakers are connected to at least one of said tandem mounted switch units, a substation selector switch, connections between each of said tandem mounted switch units and said substation selector switch, and means for connecting said first mentioned switching device to said substation selector switch whereby communication may be established between said main station and any one of said substation loud speakers.

11. In a communication system: a talk-listen switch having a call-in position, a receivinglisten position and a talk position; an amplier having an input and an output circuit, connections between said input and output circuits and said talk-listen switch; a plurality of loud-speakers positioned at various places about a building; means for selectively connecting said loud-speakers to said talk-listen switch; a call-in line between said loud-speakers and said talk-listen switch; a microphone connected to said talklisten switch; a main loud-speaker connected to said talk-listen switch; means in said talk-listen switch for connectingsaid microphone to said amplifier input circuit and means in said talklisten switch for connecting selected one or ones of said plurality oi loud-speakers to said amplifier output circuit when said talk-listen switch is in said talk position; lmeans in said talk-listen switch for connecting selected one or ones of said plurality of loud-speakers to said amplifier input circuit and means in said talk-listen switch for connecting said main loud-speaker to said ampliner output circuit when 4said talk-listen switch is in receiving listen position; and means in said talk-listen switch for connecting said call-in line to said ampliiier input circuit and for simultaneously connecting said main loudspeaker to said amplifier output circuit when said talk-listen switch is in the call-in position.

12. A communication system, comprising: a control station having sound pick-up and sound reproducing means, amplifying means including electric discharge device means having input and output circuits, and switch means for connecting said sound pick-up and sound reproducing means to the input circuit or the output circuit of said amplifying means selectively; a plurality of stations remote from said control station, sound pick-up and sound reproducing means at each of said plurality oi' remote stations, conductor means for connecting said remote stations to said switch means at said control station to enable the operal tor at said control station to talk to selected ones of said remote stations, call-in conductor means between said remote stations and said control station, and switch means at each of said remote stations for connecting selected ones of said remote stations to said call-in conductor means whereby said control station may be called from a selected remote station.

13. A communication system, comprising: a control station having a microphone, loudspeaker, and an electrical amplifier, a plurality of controlled stations remote from said control station, a loudspeaker at each of said remote stations, switch means at said control station for connecting the input of said amplifier in circuit with said microphone and the output to at least one oi said loudspeakers for the transmission of intelligence from said microphone to at least one of said loudspeakers, means associated with said switch means for connecting at least one of said loudspeakers in circuit with the input of' said amplifier and the output to the loudspeaker at said control station for the' transmission of intelligence from the loudspeaker at said remote station to said control station, and switch means at one of said remote stations for connecting the loudspeaker at said remote station to the input of said amplifier over a circuit different from the above mentioned circuits.

LOUIS W. THOMSEN. 

